1. Field of the Invention
The present invention relates to EMI reduction in switching motor drive systems, and more particularly to an active filter for EMI reduction that does not use an inductive current sensing device.
2. Background Art
In variable frequency inverter motor drive applications, and similar systems, a significant amount of common mode noise current is generated by its switching events and, in the absence of EMI filtering, flows back to the utility line. An EMI filter can be inserted to eliminate the amount of noise current that flows back to the line.
Conventional passive EMI filters for common-mode noise comprise inductors which are inserted into the line in series, and Y capacitors which are connected between the line and ground. Since the capacitance of a Y capacitor must be limited in order to limit the leakage current that flows through the Y capacitor, large common node inductors must generally be used to satisfy accepted EMI standards.
To address these problems of passive EMI filters, an active EMI filter as described herein gives an alternative solution that can potentially reduce both size and cost.
The basic idea of active common mode noise cancellation is to detect common mode noise current and duplicate it with the same amplitude but with reverse polarity, and then inject into the ground line to cancel out the noise current. As a result, all the common mode noise current from the inverter will be diverted to a closed loop created by the active filter circuitry and will not go back to the utility line.
Active EMI filters of this type are described in International Rectifier Corp.'s U.S. Pat. No. 6,636,107 and U.S. patent application Ser. Nos. 10/336,157; 10/426,123; and 10/443,686. The disclosures of the foregoing patent documents are incorporated by reference in their entirety.
However, in order to detect the common mode noise current to be canceled, the prior art has used an inductive current sensing device such as a current transformer. This kind of current sensing device, having a magnetic circuit, has flux leakage and other imperfections such as nonlinearity that restrict the overall performance of the active filter. Also, the magnetic circuit in this sort of current sensing device is bulky. Therefore, this type of active filter is not suitable to be integrated onto a silicon chip.
International Rectifier's U.S. Ser. No. 10/602,162, incorporated by reference, discloses an active EMI filter that does not require an inductive current sensing device. Further improvements in this type of filter are desired.